1. Technical Field
This invention relates to a process for converting source bit map data to monochrome data, and more particularly to convert bit map data to resolution scaled monochrome data without explicit processing of all repeating source data.
2. Background Art
Prior art processes for converting source images, usually color source data, to monochrome rasterized bit maps typically require examination of each pixel of the source bit map and conversion of the color information for each pixel to a gray scale value. After conversion, the correct dithering matrix is retrieved and scaled output pixels are generated. This is a slow, cumbersome process involving a large number of computations. This is especially so when converting visual color image data to high resolution raster bit maps, such as those utilized by laser printers having print engines capable of 300 or even 600 dots per inch. The rasterized bit map for monochrome image at 300 dots per inch resolution requires approximately a megabyte of raster memory for each letter-size page. At 600 dots per inch, 3.8 megabytes of memory are required. If the conversion process converts each individual source pixel, one at a time, and dithers each scaled group of output pixels resulting from each source pixel, processing 3.8 megabytes of rasterized bits of output image results in 30 million conversion computations and 30 million dithering operations which can take a considerable amount of time.
Laser printers typically include an on board or internal memory capable of receiving rasterized data bit map information from a host processor. This bit map is used to construct page intermediate strips of rasterized data. The page intermediate strips of rasterized data are delivered to the laser print engine which operates at a constant speed. If new rasterized data is not available at a rate that keeps up with the engine's operation, a print "overrun" occurs and the page is not printable. To prevent this from occurring, page intermediate formatting must occur at a high enough rate to keep up with the print engine. This can only be accomplished if the host processor is sending data at a high enough rate of speed, or the printer contains sufficient internal memory to hold a sufficient portion of destination image bit map which has been delivered to it at perhaps a slower rate of speed from the host processor. If the conversion, dithering and rasterization execution time, either in the random access memory (RAM) of the printer, or the host processor, is greater than the time within which page intermediate strips must be constructed for delivery to the print engine, print overruns will occur.
Execution time for prior art conversion procedures is oftentimes too slow to keep up with the construction times needed for the construction of page intermediate strips for delivery to the print engine. As a result, a number of data conversion and compression techniques are employed, usually executed in the host processor, prior to transmission of data to the printer memory to increase the rate of data transfer from the host processor to the on board memory, such that page intermediate construction can commence during the transfer while at the same time minimizing the amount of required internal printer RAM for raster memory.
Thus, in the prior art, conversion of the source color image to a rasterized bit map monochrome image has usually been completed and stored in the host processor RAM, and a compression procedure is conducted prior to transmission of the bit map image data to the on-board RAM of the printer.
What is needed is a process which speeds up the conversion process by eliminating the need to convert each individual pixel's color data information to a gray scale value where there are strings of repeating color data. Also, to increase the speed of execution time, what is needed is a process by which the requirement to dither the scaled output pixels derived from the conversion of repeating source pixels is eliminated, particularly when upscaling to a higher resolution image than the original source image. The achievement of either or both of these goals facilitates use of the conversion process, and the construction of a device independent, monochrome bit map in either the host processor, printer processor or a peripheral device processor.
Accordingly, it is an object of this invention to provide a process by which strings of repeating source pixels need not all be converted to gray scale values, and wherein all of the scaled output pixels resulting from the scaling of the source pixels need not all be individually dithered to produce a complete rasterized bit map.